1. Field of the Invention
The invention relates to magnetic bearings for rotating machines, whereby the bearing presents an integrated radial-axial design and whereby the axial control flux goes through the central opening of a soft magnetic core.
Magnetic bearings provide contactless suspension. Their low friction losses means that they are attractive for high-speed applications. However, the design of high-speed rotating machines is often complex due to rotor-dynamic limitations. In this respect each reduction of the axial length contributes to the rotor-dynamic margin. This property is applied to a maximum in ‘combo bearings’, i.e. bearings in which the design integrates axial and radial channels into a compact arrangement in which various functional parts are shared.
2. Discussion of the related art
Various examples of combo bearings can be found in patents and the literature. Frequently the path of the axial control flux crosses the central hole of a stack of laminated ferromagnetic material. Examples of this can be found in the following patents or patent applications: U.S. Pat. Nos. 5,514,924; 6,268,674; 6,727,617 which is hereby incorporated by reference; WO 2008/074045; and CN 1737388. Other examples can be found in the literature, such as in the publications of Imoberdorf e. a., Pichot e. a. and Buckner e.a. In combo bearing types as depicted in U.S. Pat. No. 6,359,357 B1 of Blumenstock, the axial control flux does not cross the central hole of a stack of laminated ferromagnetic material.
The performance of the axial channel of a combo bearing can be detrimentally influenced if the path of the axial control flux crosses the central hole of the stack of laminated material, or more generally if a combo bearing contains a section where an electrically conductive path surrounds the control flux. In that case varying control fluxes generate voltages in the surrounding material. These generated voltages cause circulation currents and consequently Joule losses, if the surrounding path is closed and electrically conductive. In fact such a stack of laminated material can be considered as a short-circuited secondary winding of a transformer, in which the axial control coil is the primary winding. The result depends on the frequency: in principle the loss increases with increasing frequency. With a certain axial control current and frequency, the Joule losses reduce the force that can be realised. As a result of this the performance of the axial channel can be affected.
Similar phenomena can occur in the stack of laminated material on which the axial actuator acts. In this case the control flux enters the stack itself, but the physical interpretation remains the same. In U.S. Pat. No. 6,268,674, Takahashi proposes making a series of equally distributed radial cuts in such a stack of laminated material. Of course the lamellae are not cut over their entire thickness, in order to maintain sufficient strength. As a result of this, induced currents remain localised, if the control flux only enters the region of the cut. This technique only provides a solution for reducing the losses in the stack of laminated material concerned. The total control flux is always surrounded by another stator stack.
The international patent application No. WO 2011/054065, describes a method for eliminating eddy currents induced in the stator stack by the axial control field. This is realised by making a cut through each lamella of the stack, and by stacking the lamellae such that the reluctance to magnetic fields within the stack is affected to a minimum. A disadvantage of this method is that the reluctance of the stack to magnetic fields that are caused by radial control coils increases. In addition, as it is known that the coating between the lamellae is not perfectly insulating, and as parts of the assembly can also lead to contact between lamellae, circulation currents can still be observed. Hence additional measures for damping these eddy currents can further improve performance.